.NET 1.1 didn't have nullable types so we added those ourself. For example NullableDateTime worked as DateTime?.When we migrated to .NET 2 and up, in order to ease the conversion process we added this using statement to avoid breaking existing code as much as possible (when replacing NullableDateTime with DateTime?). It is a leftover of course, which will be removed completely in a few years but still the R# makes a wrong suggestion ;)